(a) Field of the Invention
The present invention relates to an automatic transmission for vehicles. More particularly, the present invention relates to a shift control method for an automatic transmission which reduces shift shock occurring when shifting into back into second speed by the driver depressing the accelerator pedal during lift-foot-up shifting into the third speed from the second speed.
(b) Description of the Related Art
In the automatic transmission used for vehicles, a shift control system performs control to realize automatic shifting into different speeds and shift ranges according to various factors including throttle opening, vehicle speed and load, and several engine and driving conditions detected through a plurality of sensors. That is, based on such factors, the shift control system controls a plurality of solenoid valves of a hydraulic control system such that hydraulic flow in the hydraulic control system is controlled, resulting in the shifting of the transmission into the various speeds and shift ranges.
In more detail, when the driver manipulates a shift lever to a particular shift range, a manual valve of the hydraulic control system undergoes port conversion as a result of the manual valve being indexed with the shift lever. By this operation, hydraulic pressure supplied from a hydraulic pump selectively engages a plurality of friction elements of a gearshift mechanism according to the duty control of the solenoid valves, thereby realizing shifting into the desired shift range.
In such an automatic transmission, shift quality is determined by how smoothly the friction elements are engaged and disengaged. Namely, when changing shift ranges, the timing between the engagement of a specific set of friction elements in relation to the disengagement of another specific set of friction elements determines the shift quality of the automatic transmission. Accordingly, there have been ongoing efforts to develop improved shift control methods that enhance shift quality by better controlling the timing of friction elements to engaged and disengaged states.
The different kinds of automatic shifting operations that are performed according to driving state include sequential upshifting from the first speed to the fourth speed and sequential downshifting from the fourth speed to the first speed, skip downshifting or forced downshifting from the fourth to the second speed and from the third to the first speed, and lift-foot-up (LFU) shifting from the second speed to the third speed. The present invention relates to a method of controlling shifting back into the second speed during LFU shifting into the third speed from the second speed.
In the prior art method, when the driver depresses the accelerator pedal during LFU shifting from the second speed to the third speed, after shifting into the third speed is completed, this speed is first held for a predetermined duration before performing shifting back into the second speed. However, shift shock results from this process.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a shift control method for an automatic transmission in which when the accelerator pedal is depressed by the driver during LFU shifting into a third speed from a second speed, shifting is immediately performed back into the second speed after the completion of shifting into the third speed such that shift shock is minimized.
To achieve the above object, the present invention provides a shift control method for an automatic transmission comprising the steps of outputting a predetermined shift control signal while driving in a second speed for controlling shifting into a third speed if signals indicating that shifting from the second speed into the third speed are applied; determining if third speed synchronization is completed; performing driving in the third speed if third speed synchronization is completed, or determining if a tip-in signal has been applied if third speed synchronization is not completed; outputting a throttle compensation duty and determining if signals corresponding to 3-2 kickdown shifting are received if a tip-in signal is applied during third speed synchronization; comparing an oil temperature with a first predetermined value if signals corresponding to 3-2 kickdown shifting are received; comparing a vehicle speed with a second predetermined value if the oil temperature is greater than or equal to the first predetermined value; detecting an On/ Off state of a kickdown switch if the vehicle speed is greater than or equal to the second predetermined value; determining if a difference of turbind rpm and engine rpm and engine rpm is greater than or equal to a third predetermined value if the kickdown switch is On; performing 3-2 kickdown shift control during shifting from the second speed to the third speed if the difference of turbine rpm and engine rpm is greater than or equal to the third predetermined value; and performing 3-2 kickdown shift control after third speed synchronization if the difference of turbine rpm and engine rpm is smaller than the third predetermined value.
According to a feature of the present invention, the first predetermined value is 20xc2x0 C. and the second predetermined value is 1500 rpm.
According to another feature of the present invention, if the oil temperature is less than the first predetermined value, 3-2 kickdown shifting is performed after synchronization into the third speed is completed.
According to yet another feature of the present invention, the method further comprises the steps of determining that the kickdown switch is On if the vehicle speed is less than the second predetermined value; and performing 3-2 kickdown shift control after third speed synchronization is completed if the kickdown switch is On.
According to still yet another feature of the present invention, if it is determined that the kickdown switch is Off, 3-2 kickdown shift control during shifting from the second speed to the third speed includes the steps of outputting an SCSV-B On signal; outputting general control signals after a duty is compensated, and if an initial fill time is elapsed outputting predetermined duty control signals to perform run-up and tie-up control; and performing initial duty learn control if 3-2 kickdown shift duty control is completed.
According to still yet another feature of the present invention, the duty compensation is realized by adding a predetermined compensation duty to a final duty value of a throttle compensation duty.
According to still yet another feature of the present invention, the general control signals are output if a predetermined time has elapsed after the compensation duty is output.
According to still yet another feature of the present invention, the initial fill time is 300 ms.
According to still yet another feature of the present invention, the step of outputting the predetermined duty control signals includes the steps of performing open-loop duty control, that is, after compensating a duty Da3 by a predetermined first compensation duty following the holding of the duty for a predetermined hold time ta2, increasing a resulting duty by a rate of xcex1%/sec, and after compensating the duty by a second predetermined compensation duty, performing open-loop duty control; maintaining an output duty for a predetermined hold time te and determining if shift synchronization is completed following the completion of general control if open-loop duty control is completed; and completing duty control is shift synchronization is completed.
According to still yet another feature of the present invention, the hold time ta2 is 500 ms.
According to still yet another feature of the present invention, if the hold time ta2 is not elapsed, it is determined if the condition of Ntxe2x88x92(Noxc3x97second gear ratio) greater than 10 rpm; and if this condition is satisfied, increasing the duty by a rate of xcex1%/sec after compensating the duty by the predetermined second compensation duty.
According to still yet another feature of the present invention, if the condition of Ntxe2x88x92(Noxc3x97second gear ratio) greater than 10 rpm is not satisfied, a present duty value is compensated by a predetermined compensation value, and open-loop duty control is performed.
According to still yet another feature of the present invention, the duty output completion conditions include the satisfaction of (Noxc3x97second gear ratio)xe2x88x92Ntxe2x89xa6an eighth predetermined value for two or more cycles, or [(Noxc3x97second gear ratioxe2x88x92Nt)/dNt]xe2x89xa6a ninth predetermined value for two or more cycles.
According to still yet another feature of the present invention, the eighth predetermined value is a predetermined map value of a pre-installed program and the ninth predetermined value is 50 ms.
According to still yet another feature of the present invention, if the duty completion conditions are not satisfied, it is determined if tc has exceeded a tenth predetermined value; and if tc has exceeded the tenth predetermined value, duty is compensated by a map value in a pre-installed program, regardless of whether duty completion conditions are satisfied, then open-loop duty control is performed.
According to still yet another feature of the present invention, the tenth predetermined value is 300 ms.
According to still yet another feature of the present invention, if tc has not exceed the tenth predetermined value, duty is increased by a rate of xcex1%/sec.
According to still yet another feature of the present invention, 3-2 kickdown shift duty control completion is realized if it is determined that shift synchronization has occurred.
According to still yet another feature of the present invention, shift synchronization is determined if turbine rpm have reached a target turbine rpm.
According to still yet another feature of the present invention, the method further comprises the steps of determining if te has exceeded an eleventh predetermined time if shift synchronization has not occurred; completing shifting if te has exceeded the eleventh predetermined time, regardless of whether synchronization has occurred; and maintaining duty for te and continuously checking the completion of shift synchronization if te has not exceeded the eleventh predetermined time.
According to still yet another feature of the present invention, te is 300 ms and the third predetermined value is a map value established in a pre-installed program.
According to still yet another feature of the present invention, if it is determined that the kickdown switch if Off, a duty of 0% is output and duty control is completed.
According to still yet another feature of the present invention, the step of performing 3-2 kickdown shift control during shifting from the second speed to the third speed if the sum of turbine rpm and engine rpm is greater than or equal to the third predetermined value comprises the steps of determining turbine rpm after outputting a SCSV-B On signal and compensating duty by using a final duty value as an initial value and outputting the same; setting a duty hold time ta as a twelfth predetermined value if turbine rpm are high, setting the duty hold time ta as a thirteenth predetermined value if turbine rpm are at a medium level, and setting the duty hold time ta as a fourteenth predetermined value if turbine rpm are low; outputting general control signals if tk1 has elapsed, and determining if turbine rpm have reached target turbine rpm; setting and outputting an output duty as a map value established in a pre-installed program if turbine rpm have reached the target turbine rpm, and determining a state of the kickdown switch and performing run-up prevention control and tie-up prevention control; compensating the present output duty by xcex94d3 if the kickdown switch is Off, then increasing duty by a rate of xcex1%/sec; determining if duty output completion conditions are satisfied; compensating the duty by a compensation value established by the pre-installed program if the duty output completion conditions are satisfied, then performing feedback duty control; determining if feedback duty control completion conditions are satisfied; completing feedback duty control and completing general control if the feedback duty control completion conditions are satisfied, then maintaining output duty for a hold time te and determining if shift synchronization is completed; and completing duty control and performing initial duty learn control if shift synchronization is completed.
According to still yet another feature of the present invention, the twelfth predetermined value is 10 ms, the thirteenth predetermined value is 50 ms, and the fourteenth predetermined value is 100 ms.
According to still yet another feature of the present invention, if turbine rpm have not reached the target turbine rpm, it is determined if ta2 greater than ta1+10 ms; the output duty is set as the map value even if turbine rpm have not reached the target turbine rpm if ta2 greater than ta1+10 ms is satisfied; and it is determined if turbine rpm have reached the target turbine rpm if ta2 greater than ta1+10 ms is not satisfied.
According to still yet another feature of the present invention, if it is determined that the kickdown switch is On, it is determined if Ntxe2x88x92(Noxc3x97second gear ratio) greater than 10 rpm is satisfied; and if this condition is met, the present output duty is compensated by a predetermined value and a resulting duty is output.
According to still yet another feature of the present invention, if the condition of Ntxe2x88x92(Noxc3x97second gear ratio) greater than 10 rpm is not satisfied, the duty is compensated by a predetermined value then feedback duty control is performed.
According to still yet another feature of the present invention, the duty output completion conditions are (Noxc3x97second speed ratio)xe2x88x92Ntxe2x89xa6a fifteenth predetermined value for two or more cycles, and (Noxc3x97second speed gear ratio xe2x88x92Nt)/dNtxe2x89xa6a sixteenth predetermined value for two or more cycles.
According to still yet another feature of the present invention, the fifteenth predetermined value is a map value established by the pre-installed program and the sixteenth predetermined value is 50 ms.
According to still yet another feature of the present invention, if the duty output completion conditions are not satisfied, it is determined if tc has exceeded a seventeenth predetermined value; if tc has exceeded the seventeenth predetermined value, duty is compensated by a map value established in the pre-installed program and a resulting duty output; and open-loop duty control is performed.
According to still yet another feature of the present invention, the seventeenth predetermined value is 300 ms and if tc has not exceeded the seventeenth predetermined value, duty is increased by a rate of xcex1%/sec.
According to still yet another feature of the present invention, the feedback duty control completion conditions include Ntxe2x88x92Noxc3x97second gear ratioxe2x89xa6xc2x1XG for five or more cycles, or feedback control time (tFb)xe2x89xa7an eighteenth predetermined value.
According to still yet another feature of the present invention, XG is a map value established by the pre-installed program and the eighteenth predetermined value is 300 ms.
According to still yet another feature of the present invention, it is determined that shift synchronization is completed when turbine rpm have reached target turbine rpm.
According to still yet another feature of the present invention, if shift synchronization is not completed, it is determined if te has exceeded a nineteenth predetermined value; shifting is completed regardless of whether shift synchronization has taken place if te has surpassed the nineteenth predetermined value; and duty is maintained for te and shift synchronization completion is continuously detected if te has not surpassed the nineteenth predetermined value.
According to still yet another feature of the present invention, the nineteenth predetermined value is 300 ms.
According to still yet another feature of the present invention, run-up prevention control comprises the steps of determining if a run-up prevention start condition is satisfied; compensating a present duty by a map value and outputting a resulting duty if the run-up prevention start condition is satisfied, and determining if run-up prevention discontinue conditions are satisfied; and outputting a duty as a map value established in the pre-installed program if the run-up prevention discontinue conditions are satisfied.
According to still yet another feature of the present invention, the run-up prevention start condition includes determining if a change in turbine rpm is greater than or equal to a predetermined twentieth value.
According to still yet another feature of the present invention, the predetermined twentieth value is 40 rpm.
According to still yet another feature of the present invention, the run-up prevention discontinue conditions include determining if Ntxe2x88x92Noxc3x97second gear ratio greater than a predetermined standard value, change in turbine rpm less than 0, and Nt greater than No are satisfied, or if the single condition of the change in turbine rpm less than a predetermined standard value.
According to still yet another feature of the present invention, tie-up prevention control comprises the steps of determining if a tie-up prevention start condition is satisfied; compensating a present duty by a map value and outputting a resulting duty if the tie-up prevention start condition is satisfied, and determining if tie-up prevention discontinue conditions are satisfied; and outputting a duty as a map value established in the pre-installed program if the tie-up prevention discontinue conditions are satisfied.
According to still yet another feature of the present invention, the tie-up prevention start condition includes determining if Noxc3x97second gear ratioxe2x88x92Nt greater than a predetermined twenty-first value.
According to still yet another feature of the present invention, the predetermined twenty-first value is 20 rpm.
According to still yet another feature of the present invention, the tie-up prevention discontinue conditions include determining if Noxc3x97second gear ratioxe2x88x92Nt greater than a predetermined standard value, and a change in turbine rpm dNt less than predetermined value, or if the single condition of Noxc3x97second gear ratioxe2x88x92Ntxe2x89xa6a predetermined standard value.
According to still yet another feature of the present invention, the initial duty learn control is performed by learning a value derived by adding a duty compensation amount learned previously to a map value established by the pre-installed program.